1. Field of the Invention
The present invention generally relates to a digital playing apparatus for variable input signal clock rate which can be applied to an optical recording and/or reproducing apparatus using an optical disc, a magnetic disc recording and/or reproducing apparatus and a recording and/or reproducing apparatus using a magnetic tape. More particularly, this invention relates to a digital information signal reproducing apparatus which can reproduce the digital information signal while a relative speed between a recording medium and a reproducing device is switched upon reproduction.
2. Description of the Prior Art
In the prior art, a digital audio signal reproduced from a compact disc by an optical reproducing apparatus is converted into an analog audio signal and then this analog audio signal is supplied to a cassette tape recorder, thereby being recorded on a magnetic tape. On the other hand, in a cassette tape recorder, a magnetic tape is transported at a speed twice that of the normal reproduction mode to reproduce a recorded signal and the reproduced signal is recorded on a magnetic tape which is transported at a speed twice that of the normal recording mode, thereby a so-called double speed dubbing being performed.
Therefore, the following optical reproducing apparatus is proposed. That is, a compact disc is rotated at a rotational speed twice that of the normal reproduction mode to reproduce a recorded signal and the reproduced signal is supplied to a cassette tape recorder. Thus, the reproduced signal is recorded on a magnetic tape transported at a speed twice that of the normal recording mode by a cassette tape recorder or the like which is set in the recording mode at a speed twice that of the normal reproduction mode.
This previously-proposed optical reproducing apparatus capable of twice than normal speed reproduction will be described with reference to FIG. 1.
In FIG. 1, it will be seen that a digital audio signal recorded in a compact disc 1 is reproduced by an optical pickup head 2. The compact disc 1 is rotated by some suitable driving means such as a motor and so on though not shown.
A signal processing circuit 3 is adapted to error-correct and EFM (eight-to-fourteen modulation)-demodulate the digital audio signal reproduced from the compact disc 1 by the optical pickup head 2. An input terminal T.sub.1 is supplied with a speed control signal which shows the rotational speed of the compact disc 1, such as normal reproduction speed and twice normal speed. The signal processing circuit 3 also controls the switching operation of switches SW.sub.2 and SW.sub.3 which will be described later. The reproduced digital audio signal from the signal processing circuit 3 is supplied to an L/R separating circuit 6, in which it is separated to provide L (left)-channel and R (right)-channel stereo audio signals. These stereo audio signals of L and R channels are supplied through digital-to-analog (D/A) converters 7 and 8 to analog low-pass filters 12, 13 and 14, 15, respectively.
The L-channel analog audio signal from the D/A converter 7 is filtered out by the analog low-pass filter 12 having a cut-off frequency of 22.05 kHz and the analog low-pass filter 13 having a cut-off frequency of 44.1 kHz and is fed through the switch SW.sub.2 to an audio circuit 11 which will be described later.
The R-channel analog audio signal from the D/A converter 8 is filtered out by the analog low-pass filter 14 having a cut-off frequency of 22.05 kHz and the analog low-pass filter 15 having a cut-off frequency of 44.1 kHz and is then fed to the audio circuit 11. The audio circuit 11 amplifies the L-channel and R-channel analog audio signals supplied thereto from the analog low-pass filter 12 or 13 and the analog low-pass filter 14 or 15 through the switches SW.sub.2 and SW.sub.3 and supplies the thus amplified analog audio signals through output terminals T.sub.2 and T.sub.3 to speakers (not shown) and so on.
When the compact disc 1 is rotated at normal rotational speed, a sampling, frequency Fs is 44.1 kHz and a frequency band lower than substantially 22.05 kHz is allocated to the band of audio signal and a frequency spectrum of the digital audio signal is represented in FIG. 2A. A hatched portion in FIG. 2A represents the band of audio signal. At that time, the signal processing circuit 3 controls the switches SW.sub.2 and SW.sub.3 so that they are connected to the sides of the analog low-pass filters 12 and 14. As a result, the L-channel and R-channel analog audio signals containing high frequency components from the D/A converters 7 and 8 are respectively filtered out by the analog low-pass filters 12 and 14, each of which has a frequency characteristic shown in FIG. 2D, and are then fed to the audio circuit 11.
When the compact disc 1 is rotated at a rotational speed twice as high as the normal rotational speed, a, sampling frequency 2Fs becomes 88.2 kHz and the frequency band substantially lower than 44.1 kHz is allocated to the band of audio signal. A frequency spectrum of the digital audio signal at that time is represented in FIG. 2B. A hatched portion in FIG. 2B represents the band of audio signal. At that time, the signal processing circuit 3 controls the switches SW.sub.2 and SW.sub.3 so that they are connected to the sides of the analog low-pass filters 13 and 15. As a result, the L-channel and R-channel analog audio signals containing high frequency components from the D/A converters 7 and 8 are respectively filtered out by the analog low-pass filters 13 and 15, each of which has a frequency characteristic shown in FIG. 2C and are then fed to the audio circuit 11.
As is clear from the foregoing, if the audio signal reproduced at the speed twice than the normal speed by the above-described optical reproducing apparatus is supplied to the tape recorder so that the reproduced audio signal is recorded on the magnetic tape which is transported at the speed twice the running speed of the normal recording mode, the so-called twice normal speed dubbing can be carried out.
Further, Japanese Patent Laid-Open Gazette No. 1-175309 describes a digital signal processing apparatus in which a digital signal is obtained by changing a desirable sampling frequency and a digital signal is converted to an analog signal and vice versa.
In the optical reproducing apparatus as described above by way of example, when the compact disc is rotated at the normal rotational speed and at the twice than normal rotational speed thereby to reproduce the recorded signal, the L-channel and R-channel analog audio signals require the analog low-pass filters each having a cut-off frequency of 22.05 kHz and the analog low-pass filters each having cut-off frequency of 44.1 kHz, which unavoidably provides a complicated circuit arrangement and an expensive optical reproducing apparatus.